Ataxia-telangiectasia (A-T) is an autosomally recessive human genetic disease characterized by pleiotropic defects, including greatly increased cancer risk, thymus hypoplasia and immunodeficiency. We have constructed a mouse model for A-T by disruption of the mouse ATM gene whose human counterpart is mutated in all A-T patients tested. The ATM- /- mice recapitulate most of the A-T defects, including greatly increased incidence of lymphoid tumors and immune defects. The overall objective of this proposal is to employ genetic and molecular methods to reveal the basis of the immune defects and high incidence of lymphoid tumors in the ATM-/- mice and affected patients. An increased frequency of chromosomal translocations and inversion involving the immunoglobulin or T cell receptor (TCR) loci, possibly as a result of illegitimate joining of these loci during V(D)J recombination, may cause the high incidence of lymphoid tumors in both A-T patients and ATM-/- mice through deregulated expression of protooncogenes. To test the involvement of the V(D)J recombination in the greatly increased incidence of thymic lymphomas in ATM-/-mice, the mouse ATM mutation will be introduced into the RAG-2-/-genetic background which is completely deficient in V(D)J recombination. If the V(D)J recombination is important for the lymphoid tumorigenesis in ATM-/- mice, this tumorigenesis should be suppressed or delayed in the ATM-/-RAG-2-/- genetic background. In addition, cytogenetic and molecular analysis of the lymphomas derived from ATM-/- mice will be performed to identify common chromosomal translocations involving known or potential oncogene loci in these lymphomas. The ATM-/- mice exhibit immune defects similar to those seen in A-T patients. To test whether the reduction of thymocytes in ATM-/- mice might be due to defects in cellular proliferation or disrupted V(D)J recombination or both, a functional TCRalphabeta chain will be introduced into the ATM-/- mice. These ATM- /-TCRalphabeta+ mice will also be used to study the basis of defective T-dependent immune responses in ATM-/- mice. ATM might play two potential roles in the V(D)J recombination: one role, similar to that of DNA-PK, involves the repair of DNA double-stranded breaks during V(D)J recombination; the other involves the cell-cycle regulation during V(D)J recombination. The lymphocytes derived from ATM-/- mice will be used to test these potential roles of ATM in the V(D)J recombination.